This invention relates to an encoding technique suitable for high-speed local area network communications. It is contemplated that serial information is to be transmitted at data rates in excess of 100 MBaud over media and distances of interest (up to 50 meters on unshielded twisted pair cabling or Type "D" inside wiring (DIW), and up to 200 meters on Type 1 shielded cabling). The particular field of application is in Fiber Distributed Data Interface (FDDI).
FDDI is a high-speed Token Ring LAN protocol used primarily on fiber optic networks at data rates in excess of 100 MBaud. According to the FDDI Physical Media Dependent (PMD) layer protocol, encoding is Non Return to Zero Invert (NRZI) with a 4-bit to 5-bit conversion/translation at 125 MBaud which guarantees no more than a 14% deviation from a reference d.c. level.
FDDI allows two types of network stations or nodes on an electrically-connected ring. One class, Class A, may couple to a primary and a redundant secondary ring simultaneously. Another class, Class B, may be coupled to only one ring at a time. Class B nodes use a single connection to carry both incoming and outgoing lines of a ring. However, because of the dual fiber configuration of the primary ring, Class A nodes and Class B nodes can be interconnected only through an interfacing device referred to as a wiring concentrator. The wiring concentrator provides connection points into the primary ring that are suitable for Class B type connections.
The connection between Class B nodes and the wiring concentrator may be optical fiber or copper twisted pair wire, such as IBM Type 1 shielded wire. It would be advantageous to use Data Grade high twist wire or conventional telephone DIW unshielded twisted pair, particularly in a building already wired with such wiring. However, it has been considered difficult, if not impossible, to use such wiring at data rates comparable to the fiber optic speeds of 100 MBaud. As a consequence, there is a constraint on the rate of data exchange which prevents the use of such wiring in the connection between a Class B node and a data concentrator. Due to near-end crosstalk and attenuation, it is necessary to reduce high-frequency components in the modulated signal. Moreover, due to impulse noise, it is necessary to increase power levels to achieve the bit error rate of 10.sup.-12 BER necessary for reliable data transfer. However, FCC standards require that emission levels be limited in the range of 30 MHz to 1 GHz. Current FDDI modulation schemes cannot meet these constraints on unshielded twisted pair wiring. What is needed is a scheme for communication of baseband signals on copper wiring which is capable of data rates comparable to that of fiber optic cables while satisfying the stated constraints.